Backfires...re-jetting...corn syrup and octane

I'll start with the corn syrup...

I live in the San Francisco Bay Area, and with only 10% ethanol gas and single hose gas pumps, my bike never gets to run on 91 octane even if I hit the button because of the gallon and a half of 87 still in the hose. The other day I conned my girlfriend, I told her I would fill up her bone stock 2012 DR650 if she followed me down to the gas station...hit the 91 button, filled her up then put that pure 91 in my bike :devil: maybe it was the placebo effect, but I felt like the drz ran the best it ever has.

I told that story first because I'm not sure if that has anything to do with the backfiring. Obviously I'm pretty sure I need to re-jet it since its popping ( not on decel like normal, but when I rev it up to 1/4 to 1/2 throttle). The only other differences are cold weather (between 35 and 60 F) and I took the silencer out of the muffler.

Didn't backfire once with the 91 octane. I have the old style slant body FCR(no choke), full yoshi TRC Ti (no silencer), 3x3, no toil foam filter, elevation: right about sea level. Could somebody hit me with the proper jetting specs?

Also, one more incredibly stupid question for you guys to have a laugh about: how do I know if one of the previous owners installed hot cams or a big bore :blush: I mean it is possible the bike has these upgrades and I have no idea...the guy I bought it from rode it about 400 miles and didn't know much about it. But somebody who owned it before him put on a $750 exhaust pipe, new pumper carb, MCCT, and the 3x3 mod is REALLY clean (it's not some hack job, very pro). I'm just saying that somebody who owned this bike put some money into it and had mechanical knowledge...wouldn't it be that surprising to find hot cams or a big bore hidden in there? LOL

End rant :applause:

if there aren't any markings on the jug, then a good way to find out is to open it up. Could also do all the loctite fixes while it's opened up

or just keep telling yourself it's got all the aftermarket parts on it, to save yourself some money :thumbsup:

Edited by KaiserRolls

I'd pull the valve cover and check the cams...

As for if it has Big Bore, I don't believe there is a way to tell except for pulling the head off.

if there aren't any markings on the jug, then a good way to find out is to open it up. Could also do all the loctite fixes while it's opened up

or just keep telling yourself it's got all the aftermarket parts on it, to save yourself some money :thumbsup:

Lol...well the thought only occurred to me the other day when I had it running on the higher octane and it seemed happy for the first time in its life. I read that the drz with the big bore runs best on higher octane so that got me thinking :thinking:

Also one of the previous owners was a thumpertalk member...I deduced this theory by the MCCT, thumper talk case savers and skid plate, 3x3, and the red loctite on the counter shaft splines. Since I know how you guys are with milking power out of these bikes it's just a matter of $$$...and if he could afford to put on a $750 exhaust, I'm thinking that a big bore and cams is almost a no brainer with that kind of bank roll

I guess I'll find out when it's time to adjust the valves...

When I bought my ltz400 it just had a full pro circuit t4 exhaust. Than the valves took a shit so as I was taking the head apart I notices the cams didn't look like the stock ones. Turns out it had oshc hot cams in it. It didn't have a big bore kit though, I figured that out by taking a caliper and measuring the cylinder in mm. The bike sounded a lot like a modded 450, revved quick and had a lot of power for 400cc's. It also had an edelbrock carb which I hated, don't buy them.

I guess my main concern is how will I know what specs to use to re-jet the carb since I don't know if there is a big bore and/or hot cams hiding in there

I'll start with the corn syrup...

I live in the San Francisco Bay Area, and with only 10% ethanol gas and single hose gas pumps, my bike never gets to run on 91 octane even if I hit the button because of the gallon and a half of 87 still in the hose. The other day I conned my girlfriend, I told her I would fill up her bone stock 2012 DR650 if she followed me down to the gas station...hit the 91 button, filled her up then put that pure 91 in my bike :devil: maybe it was the placebo effect, but I felt like the drz ran the best it ever has.

I told that story first because I'm not sure if that has anything to do with the backfiring. Obviously I'm pretty sure I need to re-jet it since its popping ( not on decel like normal, but when I rev it up to 1/4 to 1/2 throttle). The only other differences are cold weather (between 35 and 60 F) and I took the silencer out of the muffler.

Didn't backfire once with the 91 octane. I have the old style slant body FCR(no choke), full yoshi TRC Ti (no silencer), 3x3, no toil foam filter, elevation: right about sea level. Could somebody hit me with the proper jetting specs?

Also, one more incredibly stupid question for you guys to have a laugh about: how do I know if one of the previous owners installed hot cams or a big bore :blush: I mean it is possible the bike has these upgrades and I have no idea...the guy I bought it from rode it about 400 miles and didn't know much about it. But somebody who owned it before him put on a $750 exhaust pipe, new pumper carb, MCCT, and the 3x3 mod is REALLY clean (it's not some hack job, very pro). I'm just saying that somebody who owned this bike put some money into it and had mechanical knowledge...wouldn't it be that surprising to find hot cams or a big bore hidden in there? LOL

End rant :applause:

Jetting as per this thread... http://www.thumperta...arberatur-help/

The better performance from the higher octane fuel could be related to the fact that the weather in your part of the world is cold...the denser fuel will richen your jetting slightly...

I guess my main concern is how will I know what specs to use to re-jet the carb since I don't know if there is a big bore and/or hot cams hiding in there

If you do have a big bore the only part of the jetting that will change is the main jet...

Assuming it's stock bore try a 160mj 1st.....Then change to a 155 mj and note any changes in performance (155 is recommended for BB)

If you do have a big bore the only part of the jetting that will change is the main jet...

Assuming it's stock bore try a 160mj 1st.....Then change to a 155 mj and note any changes in performance (155 is recommended for BB)

Thanks Craig...the silencer wouldn't change the airflow enough to merit a re-jet and cause the backfiring?

I believe that the silencer change may be the cause of the backfire, I'm not familiar with that bike but have seen it many times on other bikes. inho

Thanks Craig...the silencer wouldn't change the airflow enough to merit a re-jet and cause the backfiring?

If jetted correctly for a cork and current atmostheric conditions you will run leaner without it installed.....when removing the insert your bike will need more fuel ....

depending on climatic conditions a re jet is usually required...remember in extreme cold your bike is going to run leaner, thus insert removal will add to a lean condition..

back firing is usually caused by an air leak at the front pipe/ silencer joint, but also could be a loose carb, or exhaust at the cylinder head.

back firing is usually caused by an air leak at the front pipe/ silencer joint, but also could be a loose carb, or exhaust at the cylinder head.

It's not the gun shot backfire that happens sometimes when you turn off an old lawn mower or dirtbike, it's an un smooth rev when you twist the throttle from 1/4 to 1/2 throttle. I'm gunna put the silencer back in before I head to work today and ill report back if that fixes it. In the meantime, I'm gunna save my pennies for a K&N air filter and re-jet the carb with the new air filter and no silencer.

just got to work...with the silencer in, smooth ride...jetted perfectly with the cork

IS the only reason you think it is a big bore is because it runs better on 91 octane and you read big bores needed 91 octane? ALL DRZs E & Kick Start need at least 91 octane. It's in the manual. I know the whole story about not using higher octane than required but i run 93/94 in my CRF 250X and mt DRZ 400E Shell if I can get it. That way I know at least that part of the performance equation is covered. The DRZ400E air box engineer should get a kiss AND a GOLD STAR for making the snorkel part of a 3 inch square removable rubber plug that can be removed and reinstalled in seconds.

IS the only reason you think it is a big bore is because it runs better on 91 octane and you read big bores needed 91 octane? ALL DRZs E & Kick Start need at least 91 octane. It's in the manual. I know the whole story about not using higher octane than required but i run 93/94 in my CRF 250X and mt DRZ 400E Shell if I can get it. That way I know at least that part of the performance equation is covered. The DRZ400E air box engineer should get a kiss AND a GOLD STAR for making the snorkel part of a 3 inch square removable rubber plug that can be removed and reinstalled in seconds.

No, the 91 octane was not the only reason for my suspicion of a big bore. Basically, the bike is a 2001 I bought it with 1800 miles last October. Somebody owned this bike, barely rode it, and sunk a ton of money into it. Most of the mods are thumpertalk recommended mods and the bike seems to pull really hard for a 400. I used to have a plated xr400r before the drz and the 2 bikes are like night and day.

So basically I was just curious if there was any easy way to check and see if a previous owner installed a big bore and/or hot cams.

The DRZ400E air box engineer should get a kiss AND a GOLD STAR for making the snorkel part of a 3 inch square removable rubber plug that can be removed and reinstalled in seconds.

And minus points for not tuning the S/SM models to the same level as the Real E model...... :moon:

Edited by Craigo 485sm

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